Download Enzyme - Wesleyan College Faculty

Chapter 8
An Introduction to
Metabolism
PowerPoint TextEdit Art Slides for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.2 Transformations between kinetic and
potential energy
On the platform, a diver
has more potential energy.
Climbing up converts kinetic
energy of muscle movement
to potential energy.
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Diving converts potential
energy to kinetic energy.
In the water, a diver has
less potential energy.
Figure 8.3 The two laws of thermodynamics
Heat
+
H2O
Chemical
energy
(a) First law of thermodynamics: Energy
can be transferred or transformed but
neither created nor destroyed. For
example, the chemical (potential) energy
in food will be converted to the kinetic
energy of the cheetah’s movement in (b).
co2
(b) Second law of thermodynamics: Every energy transfer or transformation increases
the disorder (entropy) of the universe. For example, disorder is added to the cheetah’s
surroundings in the form of heat and the small molecules that are the by-products
of metabolism.
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Figure 8.5 The relationship of free energy to
stability, work capacity, and spontaneous change
• More free energy (higher G)
• Less stable
• Greater work capacity
In a spontaneously change
• The free energy of the system
decreases (∆G<0)
• The system becomes more stable
• The released free energy can
be harnessed to do work.
• Less free energy (lower G)
• More stable
• Less work capacity
(a) Gravitational motion. Objects (b) Diffusion. Molecules
move spontaneously from a
in a drop of dye diffuse
higher altitude to a lower one.
until they are randomly
dispersed.
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(c) Chemical reaction. In a
cell, a sugar molecule is
broken down into simpler
molecules.
Figure 8.6 Free energy changes (G) in exergonic
and endergonic reactions
Reactants
Free energy
Amount of
energy
released (∆G<0)
Energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Free energy
Products
Amount of
energy
released (∆G>0)
Energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required
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Figure 8.7 Equilibrium and work in closed and
open systems
∆G < 0
∆G = 0
(a) A closed hydroelectric system
(b) An open hydroelectric
system
∆G < 0
∆G < 0
∆G < 0
∆G < 0
(c) A multistep open hydroelectric system
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Figure 8.8 The structure of adenosine triphosphate
(ATP)
Adenine
N
O
–O
P
O–
O
P
O–
O
P
C
C
N
C
CH
HC
O
O
O
N
CH2
O
O–
H
Phosphate groups
H
H Ribose
H
OH
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NH2
OH
N
Figure 8.9 The hydrolysis of ATP
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
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Energy
Figure 8.10 Energy coupling using ATP hydrolysis
Endergonic reaction: ∆G is positive, reaction
is not spontaneous
NH2
Glu
+
Glutamic
acid
NH3
Glu
Ammonia
Glutamine
∆G = +3.4 kcal/mol
Exergonic reaction: ∆ G is negative, reaction
is spontaneous
ATP
+
H2O
ADP +
Coupled reactions: Overall ∆G is negative;
together, reactions are spontaneous
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P
∆G = –7.3 kcal/mol
∆G = –3.9 kcal/mol
Figure 8.11 How ATP drives cellular work
P
i
P
Motor protein
Protein moved
(a) Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
ATP
P
Pi
P
Solute
Solute transported
(b) Transport work: ATP phosphorylates transport proteins
P
Glu + NH3
Reactants: Glutamic acid
and ammonia
NH2
+
P
i
Glu
Product (glutamine)
made
(c) Chemical work: ATP phosphorylates key reactants
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i
Figure 8.12 The ATP cycle
ATP hydrolysis to
ADP + P i yields energy
ATP synthesis from
ADP + P i requires energy
ATP
Energy from catabolism
(exergonic, energy yielding
processes)
ADP + P
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i
Energy for cellular work
(endergonic, energyconsuming processes)
Figure 8.13 Example of an enzyme-catalyzed
reaction: hydrolysis of sucrose by sucrase
CH2OH
CH2OH
O
O
H H
H
H
OH
H HO
O
+
HO
CH2OH
H
OH
Sucrase
H2O
OH H
CH2OH
O H
H
H
OH H
OH
HO
H
OH
CH2OH
O
HO
H HO
H
CH2OH
OH H
Sucrose
Glucose
Fructose
C12H22O11
C6H12O6
C6H12O6
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Figure 8.14 Energy profile of an exergonic reaction
The reactants AB and CD must absorb
enough energy from the surroundings
to reach the unstable transition state,
where bonds can break.
A
B
C
D
Bonds break and new
bonds form, releasing
energy to the
surroundings.
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
∆G < O
C
D
Products
Progress of the reaction
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Figure 8.15 The effect of enzymes on reaction rate.
Course of
reaction
without
enzyme
EA
without
enzyme
Free energy
EA with
enzyme
is lower
Reactants
∆G is unaffected
by enzyme
Course of
reaction
with enzyme
Products
Progress of the reaction
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Figure 8.16 Induced fit between an enzyme and
its substrate
Substrate
Active site
Enzyme- substrate
complex
Enzyme
(a)
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(b)
Figure 8.17 The active site and catalytic cycle of
an enzyme
1 Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates
Enzyme-substrate
complex
6 Active site
is available for
two new substrate
molecules.
Enzyme
5 Products are
Released.
Products
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2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
3 Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
4 Substrates are
Converted into
Products.
Figure 8.18 Environmental factors affecting
enzyme activity
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
Rate of reaction
Optimal temperature for
typical human enzyme
0
60
40
20
80
100
Temperature (Cº)
(a) Optimal temperature for two enzymes
Rate of reaction
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4 5
pH
Optimal pH
for trypsin
(intestinal
enzyme)
6
OptimalpH
for two
two enzymes
enzymes
(b) Optimal
pH for
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7
8
9 10
Figure 8.19 Inhibition of enzyme activity
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Competitive
inhibitor
(b) Competitive inhibition
Noncompetitive inhibitor
(c) Noncompetitive inhibition
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Figure 8.20 Allosteric regulation of enzyme activity
Allosteric enyzme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Allosteric activater
stabilizes active from
Activator
Active form
Stabilized active form
Oscillation
Allosteric activater
stabilizes active form
NonInactive form Inhibitor
functional
active
site
Stabilized inactive
form
(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors
dissociate when at low concentrations. The enzyme can then oscillate again.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Binding of one substrate molecule to
active site of one subunit locks
all subunits in active conformation.
Substrate
Inactive form
Stabilized active form
(b) Cooperativity: another type of allosteric activation. Note that the
inactive form shown on the left oscillates back and forth with the active
form when the active form is not stabilized by substrate.
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Figure 8.21 Feedback inhibition in isoleucine
synthesis
Active site
available
Initial substrate
(threonine)
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off
Enzyme 2
Intermediate B
Enzyme 3
Intermediate C
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
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Figure 8.22 Organelles and structural order in
metabolism
Mitochondria,
sites of cellular respiration
1 µm
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